So you’ve lived here all your life — in fact, everyone has — but what do you really know about the Milky Way galaxy? Sure, you know it’s a spiral, and it’s 100,000 light years across. And of course, BABloggees are smarter, more well-read, and better looking than the average population, but be honest: do you know all ten of these things? Really?

Liar.

So let’s see if these really are Ten Things You Don’t Know About the Milky Way Galaxy.

1) It’s a barred spiral.

You might know that the Milky Way is a spiral galaxy, perhaps the most beautiful galaxy type. You’ve seen ’em: majestic arms sweeping out from a central hub or bulge of glowing stars. That’s us. But a lot of spirals have a weird feature: a rectangular block of stars at the center instead of a sphere, and the arms radiate away from the ends of the block. Astronomers call this block a bar, and, you guessed it: we have one.

Is fact, ours is pretty big. At 27,000 light years end-to-end, it’s beefier than most bars. Of course, space is a rough neighborhood. Who wouldn’t want a huge bar located right downtown?

By the way, the image above is not a photograph, it’s a drawing– there’s no way to get outside the galaxy and take a picture like this looking back. It would be a loooong walk home! Click the picture to embiggen and get more details (which is true for all the pictures in this post).

2) There’s a supermassive black hole at its heart.

At the very center of the Galaxy, right at its very core, lies a monster: a supermassive black hole.

We know it’s there due to the effect of its gravity. Stars very near the center — some only a few dozen billion kilometers out — orbit the center at fantastic speeds. They scream around their orbits at thousands of kilometers per second, and their phenomenal speed betrays the mass of the object to which they’re enthralled. Applying some fairly basic math, it’s possible to determine that the mass needed to accelerate the stars to those speeds must tip the cosmic scales at four million times the mass of the Sun! Yet in the images, nothing can be seen. So what can be as massive as 4,000,000 Suns and yet not emit any light?

Right. A black hole.

Even though it’s huge, bear in mind that the Galaxy itself is something like 200 billion solar masses strong, so in reality the black hole at the center is only a tiny fraction of the total mass of the Galaxy. And we’re in no danger of plunging into it: after all, it’s 250,000,000,000,000,000 kilometers away.

It’s thought now that a supermassive black hole in the center of a galaxy forms along with the galaxy itself, and in facts winds blown outward as material falls in affects the formation of stars in the galaxy. So black holes may be dangerous, but it’s entirely possible the Sun’s eventual birth — and the Earth’s along with it — may have been lent a hand by the four million solar mass killer so far away.

3) It’s a cannibal.

Galaxies are big, and have lots of mass. If another, smaller galaxy passes too close by, the bigger galaxy can rip it to shreds and ingest its stars and gas.

The Milky Way is pretty, but it’s savage, too. It’s currently eating several other galaxies. They’ve been ripped into long, curving arcs of stars that orbit the center of the Milky Way. Eventually they’ll merge completely with us, and we’ll be a slightly larger galaxy. Ironically though, the galaxies add their mass to ours, making it more likely we’ll feed again. Eating only makes galaxies hungrier.

4) We live in a nice neighborhood…

The Milky Way is not alone in space. We’re part of a small group of nearby galaxies called — get ready to be shocked — the Local Group. We’re the heaviest guy on the block, and the Andromeda galaxy is maybe a bit less massive, though it’s actually spread out more. The Triangulum galaxy is also a spiral, but not terribly big, and there are other assorted galaxies dotted here and there in the Group. All together, there are something like three dozen galaxies in the Local Group, with most being dinky dwarf galaxies that are incredibly faint and difficult to detect.

5) … and we’re in the suburbs.

The Local Group is small and cozy, and everyone makes sure their lawns are mowed and houses painted nicely. That’s because if you take the long view, we live in the suburbs. The big city in this picture is the Virgo Cluster, a huge collection of about 2000 galaxies, many of which are as large or larger than the Milky Way. It’s the nearest big cluster; the center of it is about 60 million light years away. We appear to be gravitationally bound to it; in other words, we’re a part of it, just far-flung. The total mass of the cluster may be as high as a quadrillion times the mass of the Sun.

6) You can only see 0.000003% percent of it.

When you got out on a dark night, you can see thousands of stars. But the Milky Way has two hundred billion stars in it. You’re only seeing a tiny tiny fraction of the number of stars tooling around the galaxy. In fact, with only a handful of exceptions, the most distant stars you can readily see are 1000 light years away. Worse, most stars are so faint that they are invisible much closer than that; the Sun is too dim to see from farther than about 60 light years away… and the Sun is pretty bright compared to most stars. So the little bubble of stars we can see around us is just a drop in the ocean of the Milky Way.

7) 90% of it is invisible.

When you look at the motions of the stars in our galaxy, you can apply some math and physics and determine how much mass the galaxy has (more mass means more gravity, which means stars will move faster under its influence). You can also count up the number of stars in the galaxy and figure out how much mass they have. Problem is, the two numbers don’t match: stars (and other visible things like gas and dust) make up only 10% of the mass of the galaxy. Where’s the other 90%?

Whatever it is, it has mass, but doesn’t glow. So we call it Dark Matter, for lack of a better term (and it’s actually pretty accurate). We know it’s not black holes, dead stars, ejected planets, cold gas — those have all been searched for, and marked off the list — and the candidates that remain get pretty weird (like WIMPs). But we know it’s real, and we know it’s out there. We just don’t know what it is. Smart people are trying to figure that out, and given the findings in recent years, I bet we’re less than a decade from their success.

8) Spiral arms are an illusion.

Well, they’re not an illusion per se, but the number of stars in the spiral arms of our galaxy isn’t really very different than the number between the arms! The arms are like cosmic traffic jams, regions where the local density is enhanced. Like a traffic jam on a highway, cars enter and leave the jam, but the jam itself stays. The arms have stars entering and leaving, but the arms themselves persist (that’s why they don’t wind up like twine on a spindle).

Just like on highways, too, there are fender benders. Giant gas clouds can collide in the arms, which makes them collapse and form stars. The vast majority of these stars are faint, low mass, and very long-lived, so they eventually wander out of the arms. But some rare stars are very massive, hot, and bright, and they illuminate the surrounding gas. These stars don’t live very long, and they die (bang!) before they can move out of the arms. Since the gas clouds in the arms light up this way, it makes the spiral arms more obvious.

We see the arms because the light is better there, not because that’s where all the stars are.

9) It’s seriously warped.

The Milky Way is a flat disk roughly 100,000 light years across and a few thousand light years thick (depending on how you measure it). It has the same proportion as a stack of four DVDs, if that helps.

Have you ever left a DVD out in the Sun? It can warp as it heats up, getting twisted (old vinyl LPs used to be very prone to this). The Milky Way has a similar warp!

The disk is bent, warped, probably due to the gravitational influence of a pair of orbiting satellite galaxies. One side of the disk is bent up, if you will, and the other down. In a sense, it’s like a ripple in the plane of the Milky Way. It’s not hard to spot in other galaxies; grab an image of the Andromeda galaxy and take a look. At first it’s hard to see, but if you cover the inner part you’ll suddenly notice the disk is flared up on the left and down on the right. Andromeda has satellite galaxies too, and they warp its disk just like our satellite galaxies warp ours.

As far as I can tell, the warp doesn’t really affect us at all. It’s just a cool thing you may not know about the Milky Way. Hey, that would make a good blog entry!

10) We’re going to get to know the Andromeda galaxy a lot better.

Speaking of Andromeda, have you ever seen it in the sky? It’s visible to the naked eye on a clear, dark, moonless night (check your local listings). It’s faint, but big; it’s four or more degrees across, eight times the apparent size of the Moon on the sky.

If that doesn’t seem too big, then give it, oh, say, two billion years. Then you’ll have a much better view.

The Andromeda Galaxy and the Milky Way are approaching each other, two cosmic steam engines chugging down the tracks at each other at 200 kilometers per second. Remember when I said big galaxies eat small ones? Well, when two big galaxies smack into each other, you get real fireworks. Stars don’t physically collide; they’re way too small on this scale. But gas clouds can, and like I said before, when they do they form stars. So you get a burst of star formation, lighting up the two galaxies.

In the meantime, the mutual gravity of the two galaxies draw out long tendrils from the other, making weird, delicate arcs and filaments of stars and gas. It’s beautiful, really, but it indicates violence on an epic scale.

Eventually (it takes a few billion years), the two galaxies will merge, and will become, what, Milkomeda? Andromeway? Well, whatever, they form a giant elliptical galaxy when they finally settle down. In fact, the Sun will still be around when this happens; it won’t have yet become a red giant. Will our descendants witness the biggest collision in the history of the galaxy?

That’s cool to think about. Incidentally, I talk about this event a whole lot more, and in a lot more detail, in my upcoming book Death from the Skies! In case you forgot about that.

Until then, these Ten Things should keep you occupied. And of course, I only wanted to list ten things so I could give this post the cool title. But if there’s something you find surprising about the Milky Way, leave a comment! I don’t want to hog all the fun.

Thanks for some cool facts. On a clear dark night out in the country, I’m always amazed at the sight of the Milky Way.

Whether looking at the Milky Way naked eye, through binoculars or through a telescope, there’s always that milky, cloudlike haze. A telescope resolves the cloud into millions of stars, but I take it the milky effect is due to us looking edge-on into many stars in the galaxy.

Since I was a kid I read that we can’t see the center due to dark interstellar clouds. Though they’ve now detected the effects of the massive black hole there.

“Andromeda is the largest galaxy of the Local Group, which consists of the Andromeda Galaxy, the Milky Way Galaxy, the Triangulum Galaxy, and about 30 other smaller galaxies. Although the largest, it may not be the most massive, as recent findings suggest that the Milky Way contains more dark matter and may be the most massive in the grouping.[7] However, recent observations by the Spitzer Space Telescope revealed that M31 contains one trillion (1012) stars, greatly exceeding the number of stars in our own galaxy.[8] 2006 estimates put the mass of the Milky Way to be ~80% of the mass of Andromeda, which is estimated to be 7.1 × 10^11 solar masses.[2]”

So the Andromeda galaxy is 20 percent more massive but it has 5 times the number of stars as the Milky Way. What gives there? Is each star on average much lighter in the Andromeda galaxy than the stars in the Milky Way?

This comment was deleted by me because it was to a post that got lost when the server moved, and unfortunately I didn’t make a copy of it. I don’t want this particular post derailed by someone railing against science when it is far off-topic.

I’m sorry, I don’t like deleting comments, but in this case the comment itself makes little sense without the context of the lost post.

“Another reference frame is provided by the Cosmic microwave background (CMB). The Milky Way is moving at around 552 km/s[37] with respect to the photons of the CMB. This can be observed by satellites such as COBE and WMAP as a dipole contribution to the CMB, as photons in equilibrium at the CMB frame get blue-shifted in the direction of the motion and red-shifted in the opposite direction.”

Loony: oops. That’s what I get for cranking out a post in like an hour, then waiting a few minutes to edit it. 😉 Fixed.

Tom: the masses of the two galaxies aren’t clear — they’re very hard to get accurate — but last I heard Andromeda is slightly less massive than us, but spread more (bigger). Note that the wikipedia entry says it’s not the most massive as well. It may have more stars, but if they are lower in mass on average than in the MW then Andromeda could have less mass even with more stars.

Our sun probably will not collide with another from the Andromeda galaxy, but is it likely that the Earth’s orbit will be perturbed by passing stars? Although, by that time, if our descendants have not increased Earth’s orbital, the sun gradual heating will make Earth way too hot for life anyway…

Just thought I’d mention that yes, I knew each of those ten things about my galaxy.

If I were to pick one as the most shocking (when I first learned it), it would be that we’re a barred spiral. Chalk it up to a couple decades of inaccurate artists’ impressions of our galaxy from the top down.

They will come in handy when some conversation at work starts to drag. The smart people will be interested and middle management will wander off in boredom. And once they are gone, we smart ones will be able to plan our coup! Bwaa-haa-haa!

Well, except I’m not 100% sure I ‘know’ that the Milky Way is going to collide with the Andromeda galaxy: I see plenty of assertions around the place that its radial velocity is towards us, but I wouldn’t mind seeing an explanation of how we can know that it has no transverse component. At that distance is it possible to measure proper motion?

Point 7 jarred after point 6: my reaction (pretending to be a naive reader) was ‘but haven’t you just told us that it’s 99.9999997% invisible?’ Only 90% is a bit of a let-down. Yeah, I understand the difference between the points you are making, but presentation counts, too.

Finally, I thought the Andromeda Galaxy has more ordinary matter but less dark matter than us.

Oh well, I don’t have any smarty-pants comments! I didn’t know any of these things, but that’s ’cause I’m a complete layman. Thanks for the most interesting blog entry I’ve read in a long time. Btw, Milkomeda for sure. Sounds way better than Andromeway.

I knew at least some of all ten (don’t memorize the numbers, though, and #9 was iffy for me) but then, I’m a physics major, astrophysics junkie and voracious reader of this site for well over a year (hm, mebbe a coupla years now), so I’ve seen it all before here. Good stuff, BA!

Must correct you on one point – not an astronomical one, but a musical one (well I am a musician, so this is my area of expertise!)

CDs do not warp in sunlight. Try it. I have never managed to warp a CD, and I’ve tried, believe me. You can boil a CD for an hour with no ill effects. Mind you, the tests I did were in the 1980’s when all CDs were factory pressed. I haven’t tested CDRs, CDRWs, DVDs, CD RAMs, etc. But I doubt they are any different when it comes to temperature. Of course they can still be ruined by a single fingernail scratch – but that’s a different matter! Leave a CD on the surface of Mercury and it might suffer a little, but here on Earth it’s quite safe in the midday sun.

As for the Milky Way – I used to like the Cadbury’s ones. Do you get them over there in the States? Delicious!

@ Tom Marking: “The Milky Way is moving at around 552 km/s[37] with respect to the photons of the CMB.”

This begs the question: Where are we going in such a hurry?

I knew 8 of the 10. I knew we were in a spiral, but not that it was barred, and not that our whole galaxy was representative of the republican party, or the Catholic priesthood, or the ‘cdesign proponists’- (warped).

I knew 9 out of 10, since I ‘sort of’ knew the mechanics behind the spiral arms, but never really thought about it much. I think maybe this post doesn’t give enough credit to your loyal blog-followers, however?

I knew all but #8 and #9. Nine made sense when I thought about it. It was #8 that really intrigued me. I have wondered about the spiral arms for years. Why some stars seem to “live” there and others like our sun only pass through them. The lightbulb is on!! Thanks, Phil. Another mystery solved!

I got 8 out of 10 — I thought Andromeda was more massive than the Milky Way (not just wider, though in my defense I will latch onto the BA’s point that it’s hard to accurately measure either galaxy’s mass that accurately), and I had no idea that our galaxy was warped.

I will definitely have to find a way of working the warped galaxy comment into a conversation, or a Dr Who fanfic. 😉

Like several others here, I knew all 10! I am not a professional astronomer, but that does not mean I’m an average layman either! I got my first telescope when I was 7 and have spent the succeeding 50+ years studying the field. I might have been a professional (that was my intent) but divorce upset those plans and I had to get a real job or starve!

the masses of the two galaxies aren’t clear — they’re very hard to get accurate — but last I heard Andromeda is slightly less massive than us, but spread more (bigger).

BA, last I heard not only was Andromeda more spread out, it was far more massive than the Milky Way. There’s always been some uncertainty about the exact mass of both galaxies, but Andromeda is definitely bigger.

Rabb, the sun’s orbital period is about 225 million years, but apparently it can’t be described very well by a Keplerian orbit. There are lots of weird effects from the highly nonzero local mass of stars; e.g. we oscillate above and below the midplane every 42 million years or so.

Also, 200 billion solar masses for the visible mass of the galaxy does not correspond to 200 billion stars in the galaxy! Stars are more numerous the lower their mass (-1.35 power if you integrate Salpeter’s ancient initial mass function), so the mean star mass is around 0.5 suns and there are 400 billion stars in the galaxy. (I tend to use 150 billion solar masses and 300 billion stars, but I’m going by Bok and Bok 1981, which has got to be way obsolete by now.)

One of my earliest and favorite t-shirts was one that had a picture of our galaxy on it, with an arrow pointing to one tiny dot in the Western Spiral Arm, and the text “YOU ARE HERE”. Funny, educational, and humbling all at the same time.

Hi Bad Astronomer,
Is it true that the gravity near the center of the Milky Way is more intense than the gravity in the outer fringes? If so, then because of relativity, does time tick ‘slower’ near the center relative to the outer areas of the Milky Way?
– archetyper

I have to admit, while I understand that the spiral arms are an illusion, I still don’t grasp how they occur. I’m not sure the traffic jam analogy works. I understand that gas and dust can slam into slower moving gas and dust and create stars that illuminate the arms. However, cars speed up again when they exit the traffic jam leaving the slower cars behind and can then run into a second traffic jam. But and I don’t suppose the stars or gas and dust speed up when they leave the arm. So how do we get the second spiral arm (as we move out from the center)? And why are the arms spiraled as opposed to concentric rings?

# Elwood Herringon 13 Mar 2008 at 5:48 am
CDs do not warp in sunlight. Try it.
===
Yes, that has been my experience as well.
They make excellent replacement chime wind catchers.
I’ve used them outside in the Texas sun for years.

Nice post, Phil! I also vote for “Milkomeda.” Any of your readers who subscribe to Astronomy magazine may have noted one of their top 10 stories for 2007 – the calculation of what will happen to our solar system when Andromeda and the Milky Way collide. Short answer: we end up way on the outskirts of Milkomeda (yes, that’s the name they used in the story). The research was the topic of a press release:http://www.cfa.harvard.edu/press/2007/pr200714.html

Also, for the person asking how we can know about Andromeda’s transverse motion (aka proper motion or motion in the plane of the sky), you can find some gory details in this research paper:http://adsabs.harvard.edu/abs/2005ApJ…633..894L

Nice. I love badastronomy.com.
Thanks for the note about your book – Death from the Skies.
I’ll buy it when it releases.

At the Hayden planetarium in NYC, I watched this cosmic collision of Andromeda and Milky Way – was mesmerizing and amazing.
They had each second represent something like 40million years or something like that. Essentially a speeded up collision of the 2 galaxies and it was beautiful to watch and awe-inspiring. Dare I say we got a better glimpse of the future than our descendents ?

Years ago I attended a company sponsored adult education class called, “Introduction to Astronomy and Astrodynamics”. In one class section we were discussing the local galaxy group and I said something about us colliding with Andromeda. The instructor, who had a PHD in astrophysics, gently pointed out to me that it should not be called the Andromeda galaxy, it should correctly be referred to as-
M31 IN Andromeda.

I passed the class anyway :o)

PS- I knew all 10 except the size and mass difference and I think that is because they have changed those figures over the years with better observations I wasnt aware of.

“Tom, BA: if visible matter (of which stars are only a part) makes only 10% of the galaxy’s mass, how could counting stars in Andromeda and Milky Way give a good prediction of their relative mass?”

I’m not sure what the 580 billion solar masses for the Milky Way and 710 billion solar masses for M31 (listed in Wikipedia) are based on. If it’s based on luminosity then it should give a rough measure of the number of stars. If it’s based on the Doppler effect due to rotation then it will include dark matter as well. Does anyone know what these numbers are based on?

But apparently the 1 trillion stars mentioned concerning M31 is presumably based on actually counting stars in some representative star field of a telescope and then multiplying by some factor. It seems that the Andromeda galaxy has far more stars than our galaxy by at least a factor of 2, maybe more.

On a sad note, it seems that NASA is looking for a new name for the Spitzer Space Telescope now that New York governor Eliot Spitzer has been caught with his pants down (like all good Democrats). How about the Phil Plait Memorial Space Telescope? Sounds good to me.

Nah, I’m sure I had some of you going there for a moment and I even wondered myself for a time. It turns out that the Spitzer Space Telescope is named after Dr. Lyman Spitzer who was an American physicist. It has nothing to do with the perv from New York.

“On a sad note, it seems that NASA is looking for a new name for the Spitzer Space Telescope now that New York governor Eliot Spitzer has been caught with his pants down (like all good Democrats). How about the Phil Plait Memorial Space Telescope? Sounds good to me.”

I too do not fully understand how the spiral arms work. Some type of shockwave is propagating in a circular direction around the Milky Way galaxy. According to Wikipedia the sun’s galactic rotation period is 220 million years, the spiral rotation period is 50 million years, and the bar rotation period is only 15-18 million years. So the bar in the barred spiral is not synched up with the main spiral pattern for some reason. It rotates 3 times faster.

And if the spirals don’t end up winding up or winding out then doesn’t the angular velocity have to be constant with distance from the center? So that would be 360 degrees per 50 million years which would be 7.2 degrees per million years. At the distance of the sun from the galactic center (26,000 light-years) the shockwave velocity would be 980 km/sec. The galactic orbital velocity of the sun is 220 km/sec which means that the spiral arm shock wave is overtaking us at 760 km/sec.

At half the distance to the center (i.e., 13,000 light-years) the shockwave velocity would be 490 km/sec. I have no idea why this would be the case (i.e., why the shockwave velocity increases with radius). The typical orbital velocity of a star about the center is usually between 210 and 240 km/sec because the mass of the galaxy is distributed and not concentrated in the center. Therefore, at a distance of 5,800 light-years from the center the shockwave velocity and the orbital velocity should equal each other. A star located here if out of the spiral arm will always be out of the spiral arm. Shouldn’t this be the true GHZ (galactic habitable zone), not where the sun is located? But man, it seems awfully close to the central bulge. Can someone please check my math.

“Is it true that the gravity near the center of the Milky Way is more intense than the gravity in the outer fringes?”

No, it’s just the opposite as a matter of fact. One of problems you solve in first year physics is what is the gravitational force on a point object located inside a spherical shell of uniform thickness and uniform density. It turns out to be zero because the different parts of the spherical shell cancel each other out for a point on the inside. The same effect should also happen in a circular disc if you are at the exact center.

If we ever send radio signals toward th Andromeda Galaxy, it should be a picture of the Andromeda Galaxy, because it would be cool if we ever detected a signal coming from Andromeda and it was a picture of the Milky Way. Just a crazy thought!

Man, this site is fantastic… One very dumb question though I am going to quote you if you don’t mind.

“By the way, the image above is not a photograph, it’s a drawing– there’s no way to get outside the galaxy and take a picture like this looking back. It would be a loooong walk home! Click the picture to embiggen and get more details (which is true for all the pictures in this post).”

How do we know the Milky way is a spiral galaxy if we have never seen it from a far? how do we know the shape of it?

Thanks

P.S. Keep up the good work! thanks for shedding light on things and the mysteries of the universe to people like me.

Although it’s true we obviously can’t get an *actual* picture from outside the galaxy looking back, we have enough data from observation that we can pretty easily extrapolate what the Milky Way would look like if we could see it from outside looking back.

We know it’s a spiral galaxy, and we have LOTS of actual images of other spiral galaxies… so renditions of the Milky Way are interpretations based on images of other spiral galaxies, adjusted for what we know to be particulars about mass, number of stars, and overall distance.

Fantastic post, BA. If you split some of these into multiple items, like “It’s a cannibal” and “It’s eating other galaxies right now,” you could get up to twenty items, and then submit it to mental_floss magazine. However, considering that that Ben Stein was a contributor in their latest issue, I might be canceling my subscription. I liked it for the trivia, but I want to make sure they don’t include lies. How can I trust a magazine that lets BS write an article.

Wow, that turned into a downer.

I think I knew (with some accuracy) about 7 of these. (The very tiny percentage of visibility struck me with awe.) Of course, I did learn most of these things by reading your blog. Keep up the great work. I need to get to a bookstore and buy your book. Hate working nights 😛

According to our friend Kepler, “The squares of the orbital periods of planets are directly proportional to the cubes of the semi-major axis of the orbits.”, so the further you are from the center, the slower you go, giving rise to the spiral shapes.

So the angular velocity is NOT constant with distance from the center, and you whole argument didn’t seem to make much sense to me.

—
And the galaxy is NOT a spherical shell of uniform thickness and density. Since gravity is inversely proportional to the square of the distance, being close to a massive black hole would seriously tug you harder then being far away, so I think archetyper’s question is a very good one.

It is too bad there is nothing to reflect our Galaxy so that we could see its true form.

As for names for the composite galaxy after the collision, I was thinking Cetus (whale/sea monster); I believe that was is the constellation that is about to eat Andromeda (before the noble Perseus comes to rescue). Whales are known for their largess as well)

> According to our friend Kepler, “The squares of the orbital periods of planets are directly proportional to the cubes of the semi-major axis of the orbits.”, so the further you are from the center, the slower you go, giving rise to the spiral shapes.

Kepler’s laws only work in a 1/r potential – the galaxy is not a 1/r potential (not by a long shot) so applying Kelpers laws is futile.

> As for names for the composite galaxy after the collision, I was thinking Cetus (whale/sea monster); I believe that was is the constellation that is about to eat Andromeda (before the noble Perseus comes to rescue). Whales are known for their largess as well)

“According to our friend Kepler, “The squares of the orbital periods of planets are directly proportional to the cubes of the semi-major axis of the orbits.”, so the further you are from the center, the slower you go, giving rise to the spiral shapes. So the angular velocity is NOT constant with distance from the center, and you whole argument didn’t seem to make much sense to me.”

If it worked like the solar system then the spirals would wind up. Imagine what would happen if you attached a string connecting Mercury to Venus to Earth to Mars and so on. Mercury would wrap around the sun faster than Venus which would wrap around faster than Earth and so. You’re string would be stretched apart and break right away. If the string was initially in a spiral pattern the pattern would be lost right away. In order to maintain a spiral structure over millions of years it cannot follow a Keplerian orbital scheme.

How well known in this number? I was watching Carl Sagan’s Cosmos and his claim was that the Milky Way contains 100 billion, and I remember from some other source that it’s 300 billion, and I see other posts in this very thread with numbers like 150 and 400 billion… I thought this number would be pinned down a little better than it appears to be.

> How well known in this number? I was watching Carl Sagan’s Cosmos and his claim was that the Milky Way contains 100 billion, and I remember from some other source that it’s 300 billion, and I see other posts in this very thread with numbers like 150 and 400 billion… I thought this number would be pinned down a little better than it appears to be.

Why? The vast majority of stars are red dwarfs – and it doesn’t really matter if there are 100 billion or 200 billion. If you counted all the animals on earth, you’d be spot on with the easy to see elephants, but your certainty on the number of ants could easly be a factor of 2, 3, 4 or more.

Oh, and for the name of the combined Milky Way/Andromeda galaxy, since it will likely end up having long tendrils of stars flying out as they collide, we should name it for its apparent shape, something like Cephalopod, or the Cthulhu galaxy. Or we could go with something like Pharyngula galaxy. Oh, that has a nice ring to it. 😉

Wow. I’m a layperson just getting into the sciences, and I knew over half of these (6). But that’s because I lurk around here. Plus, my son’s been shouting out bits and pieces from the BA book the past few days, and some of them must have hung around in my grey matter.

When I get some time, I’ll read the book myself. Inching through Sagan’s Demon-Haunted World right now.

Thanks for the great article, Phil. Re “grab an image of the Andromeda galaxy and take a look. At first it’s hard to see, but if you cover the inner part you’ll suddenly notice the disk is flared up on the left and down on the right. Andromeda has satellite galaxies too, and they warp its disk just like our satellite galaxies warp ours.” The warp of the Andromeda galaxy is kind of hard to see from this angle. Is there a photo from another perspective?

“Great post. Curious though, is it possible that the two galaxies’ super massive black holes would collide? If so, what would that result in?”

An even bigger and more super-duper-supermassive black hole! 😉

Really … What happens as I gather things is that those Central Galactic Black Holes will fall in together, throwing stars and gas-dust about everywhere, will orbit each other for a bit then fuse into one.

& from what I understand such black holes mergers have happened before and explain why they are so super-supermasive in the first place …

Oh, & just recalled, I think there just may be a chance the colliding black holes will result in a period of time where our fusing Milkomeda Galaxy becomes a quasar or AGN (Active Galactic Nucleus) for a while. It’ll be very bright at many (all?) wavelengths, shooting beams of ejected matter / energy at extremely high velocities and more … quite messy and variable .. Interesting times indeed!

Plus, that warp in our Galaxies disk – isn’t that the feature also known as ‘Gould’s Belt’ and associated with the whole Orion O-B stellar association and star forming region?

Liberal Democrat here. Tom et al., if you refrain from associating Democrats with adultery, I’ll try to refrain from pointing out that Bob Dole, Newt Gingrich, and Rudy Giuliani all divorced their wives in order to marry their girlfriends, and that some of the loudest homophobic voices in the GOP have turned out to be closet gays (e.g. Larry Craig and Ted “I can talk directly to George Bush” Haggart); being gay not being a bad thing, but hypocrisy and adultery being bad things.

[[How do we know the Milky way is a spiral galaxy if we have never seen it from a far? how do we know the shape of it?]]

We’re estimating from studies of star distribution and hydrogen nebulae and so on. We have the nearby arms (Perseus outside ours; Orion which we’re in; and Sagittarius closer to the core) fairly decently mapped from radio astronomy studies. We know where the center of the galaxy is because it’s a radio source and from the distribution of globular clusters. The fact is, though, that we don’t have a really good physical map of the galaxy at this time. We know enough to know it’s a barred spiral about 100,000 light-years in diameter and 3,000 light-years thick, the latter figure being a very crude mean of the various “scale heights” of different star populations.

OMG! A rectangular section at the center of our galaxy, where the black hole rules? Now we that we find a “cubic structure” in the middle of Earth might we also find a domestic black hole ?
It’s black holes all the way down:

“”We found that the body centred cubic structure of iron is the only
structure that could correspond to the experimental observations,”
according to Prof Brje Johansson of Uppsala University. It has long
been known that the inner core of the earth, a sphere consisting of a
solid mass with a radius of about 1,200 km, is mainly made up of iron.
But, in this study, the team has found that elastic waves pass more
rapidly through the earth’s core in directions parallel to our
planet’s axis of rotation than in directions parallel to Equator, a
phenomenon not previously explained. In fact, the researchers have
shown simulations of how seismic waves are reproduced in iron under
the conditions that prevail in earth’s core, revealing a difference of
12 per cent depending on their direction which suffices as an
explanation for the puzzling observation. First the trajectories of
movement were calculated for several million atoms in strong
interaction with each other. On this basis, the scientists were then
able to determine that the progress of the sound waves was actually
accurately described in the computer-generated model for iron under
the conditions prevailing in the core of the earth.”http://inquisitiveworld.blogspot.com/2008/02/cubical-model-of-earths-core-found.html

Did you know observing the milkyway’s black hole is easy? All you need is a short wave radio and a simple die pole antenna such as a rabbit ears. Simply attach the antenna to the radio and set the dial to 20.5 megaheartz. The radio noise is very loud and and easy to hear.

This might be a bit on the pedantic side, and I’m really not trying to correct you, Michael… so my apologies ahead of time, but I thought what you are referring to was actually the Sagitarius A radio source… which consists of three separate elements, one of which is Sagitarius A*, the supermassive black hole at the center of the galaxy (ok… well, the radio source *associated* with the supermassive balck hole, to be accurate).

Is this still correct, that the backround noise at 20.5 MHz first described by Jansky is realted to these three elements? Or has it been pretty much attributed to Sagitarius A*?

Here’s a question: you mention the “warp” of the Milky Way. As I understand it, our solar system has a sort of waveform motion relative to the galactic plane. We sort of bob up and down through the thickness of the Milky Way as we slowly cirlce it, like a ping pong ball caught in the motion of a rippling pond. Is this warpage what causes this?

Also, as we change position, does this alter our exposure to radiation from the core, or (contrariwise) decrease our exposure to possible collisions within the more crowded galactic plane?

While you are addressing cool things about the Milky Way, perhaps you could speak to, or speculate about what goes on as you approach the galactic center. How close are those stars to each other compared to the suburbs? Is the night sky lit up like Broadway? What about radiation levels? Can planetary systems be expected to survive in a stable fashion if stars are whipping around a black hole and their neighbors at such velocities? What might an observer on such a planet be likely to see? Are double-stars any more or less common in this neighborhood? Inquiring minds want to know.

Kepler’s laws ONLY apply in a 1/r potential (i.e. a planet orbiting the sun). The Mass distribution in the galaxy IS NOTHING like the mass being concentrated in a central point and so the potential is nothing like 1/r.

“Thanks for the great article, Phil. Re “grab an image of the Andromeda galaxy and take a look. At first it’s hard to see, but if you cover the inner part you’ll suddenly notice the disk is flared up on the left and down on the right. Andromeda has satellite galaxies too, and they warp its disk just like our satellite galaxies warp ours.” The warp of the Andromeda galaxy is kind of hard to see from this angle. Is there a photo from another perspective?”

Not likely! 😉

To get another perspective on M31 (the Andromeda Galxy) you’d need to move well above or below the plane of our own Galaxy – in fact you’d probably need to be well outside the MilkyWay altogether!

You may be able to see more of this at other wavelengths however … Or specially scientifically enhanced and false coloured photos – but afraid I’m none too sure. I have seen images of M31 through different filters and enhanced in dif. ways but as for seeing the warp in ’em ..? Not sure, sorry ..

Fact 13: If you have a spacecraft with a constant thrust of 9.8066 newtons per kilogram then a trip to the center of the Milky Way can be accomplished in only 19.75 years ship time. Of course, 26,000 light-years will elapse for the people left back on earth. So the blackhole at the center of the Milky Way galaxy could be visited in less than a human lifetime for the astronauts on the spacecraft. Peak velocity at the midpoint of the trip is 0.9999999972c. Here are some details:

JediBear’s scorecard:
1) Knew this. Kinda surprised when I found out we had not only a bar but a large one, but that was years ago. Sadly, I have yet to visit the bar. If local trends are any indication, it will have closed by the time I manage to arrange a visit.
2) Yep. Knew that. Heh. LGM.
3) Knew that. I’ve known it for years, but I think I learned about it more recently than the bar.
4) I’ll give myself partial credit on this. I knew there was a neighborhood, and that we were among the biggest kids in it, but I wasn’t really up on the standard of living. The white picket fences were basically news to me. Call it a 7/10
5) Fail. I was vaguely aware of the Virgo Cluster. I didn’t know we were gravitationally influenced by it. That’s kinda strange. It’s like knowing you’re in Richland and the US, but not that you’re in Washington.
6) Didn’t have an exact number, but between astronomy 101 and this site, I think I had picked up the basic idea. I’ll give myself a huge helping of partial credit here. Call it a 9/10.
7) This I knew
8) This one I hadn’t even thought about. Kind of a “duh!” moment when I read it here. Fail.
9) Nope. No idea. Fail again.
10) Yeah, I’d heard that somewhere. Didn’t know about the Andromeda Galaxy being visibly larger than the moon though. Never seen it myself or heard its size described before. Technically, that’s off-point though. Would have been extra credit maybe.

Strictly speaking, this is a grammatical english sentence. “just” means “simply” and modifies the “do” out of “don’t”(do not.) You are correct that it’s probably not what’s meant here, though it would probably suffice.

>should be – We don’t just make this stuff up you know..”

Here “just” means “merely” and modifies the objective verb “make.” This might actually be seen as a weaker statement than the above, since it implies that while we DO make these things up, that there’s some additional effort involved, while the above suggests that no creative effort is involved.

On the other hand, this is more accurate. Making stuff up and then confirming it describes the scientific process perfectly.

>What part of speech is JUST? An adverb?

In both of these cases, yes. It can, however, be an adjective, depending on which of several homographs you mean.

>~make_up(we, stuff(this))

pidgin-LISP is funny, but it took me longer to figure out what this meant than did the bit of ungrammatical english you’re replying to.

>Bah, English – a bunch of completely illogical grammatical rules. >Where’s Prolog or Lisp when we need them:

English is both logical and thoroughly comprehensible, if neither strictly phonetic, nor unambiguous, nor context-free.

Prolog and Lisp are for the most part thoroughly incomprehensible, unless you’re a compiler or interpreter.

To one of your earlier points:
>One of problems you solve in first year physics is what is the >gravitational force on a point object located inside a spherical shell of >uniform thickness and uniform density.

However, the Milky Way’s density is hardly uniform.

For one thing, /it contains a supermassive black hole/. Within certain constraints, the closer you get to that thing (and the center of the galaxy,) the stronger your gravitational experience is going to be, if you catch my meaning.

By the way, the image above is not a photograph, it’s a drawing– there’s no way to get outside the galaxy and take a picture like this looking back. It would be a loooong walk home! Click the picture to embiggen and get more details (which is true for all the pictures in this post).

The meaning of the predicate calculus expression is completely clear which is something you can’t say about the English sentence.

“For one thing, /it contains a supermassive black hole/. Within certain constraints, the closer you get to that thing (and the center of the galaxy,) the stronger your gravitational experience is going to be, if you catch my meaning.”

The original question was is there something special gravitationally speaking in being at the center of the Milky Way galaxy. The answer is no. If you took the central blackhole and moved it out to the sun’s neighborhood 26,000 light-years from the center the gravitational effects would be just as intense. It has to do with being close to the blackhole, not being close to the center. It just (there’s that word again) so happens that the blackhole is at the center of the galaxy.

So who are these idiots that think they need to post how many of these facts they knew already? Future forum directions for said morons: 1. Move on to next blog entry
– Sorry to start the flame war but I can’t stand people’s tendency to measure self-worth through the internet. Guess what, this medium makes you think you’re smarter than you really are… God, I’m smart.

I did know these 10 facts and was pleased to have someone send me your site. I have a GUT called the Elliptical Illusion where I refute both the earth AND the sun centered views. I have it copyrighted it and this is the short version:

The planets’ elliptical orbits are an illusion, they are actually complex spirals…As the Sun travels, so does Earth-Moon…Graphing this motion in 2 dimensions produces a sine wave…when 3D, it is a spiral.

I am a self-taught astronomy amateur and have perused Sites and Books and have never found anyone that has “discovered” this. I built a model once of the Sun and 1st 3 planets including the moon and took it to an Astronomer in El Paso, Texas. He told me they teach these motions in advanced courses…have I “discovered” something or did someone beat me to it?

Nice site, but I have a comment for Naoki: Who says we came from Sagitarius? A Lakota friend of mine says they came from Syrius and I know other Indigenous people claim they come from various other places. Presently we trail Vega by 25ly and are ahead of Syrius by 8ly…Do Stars “pass” eachother and drop off DNA on eachother?

Loved the article & following blogs. I thought I’d read/seen/heard that our galactic black hole just woke up. If I remember correctly…They were observing a galaxy with it’s black hole turned off, then looked at ours & saw the same?
What I got was that galactic black holes shut down, during that shut down the stars racing around the rim also slow down. Then when the Black Hole “wakes up”, that’s when we see the center suns speed up.
The spiral formation is easy to recreate by draining a sink with some suds.

This is something I’ve been pondering for quite a while. Ever since I heard about this, in fact. Thanks for posting what it is not! There is one thing I’ve been thinking about and it is not listed. That means either I am so ignorant I don’t even understand the basic principles of what I am suggesting, or there might be something to it. I’m hoping the commenteers here could shed some light unto it (pun unintended, you’ll see in the next sentence).

Couldn’t the lack of gravity come from photons “floating around”? I checked around and saw that photons didn’t have mass, they have momentum. That is a big reason I think I could be wrong. However, I read that photons react to gravity (which is why we have gravitational lensing). That tells me that either photons just *react* to gravity or momentum causes distortion in the spacetime itself. I don’t know which. It’d be cool to calculate to see how much mass a star is loosing and see if any of that could be “floating around” as light.

the sun’s orbital period is about 225 million years, but apparently it can’t be described very well by a Keplerian orbit. There are lots of weird effects from the highly nonzero local mass of stars; e.g. we oscillate above and below the midplane every 42 million years or so.

I am not sure what will happen when the milky way, sun, and earth line up but I do not think this it will be good because also the earth is going to wabble on its axis the same time, to many things are happing then. Already the earth is changing and I do not think that it is just global warming. It may be global warming but in a different sense, the earth is changing. If people were not so busy and took the time, like my grandmom told me, to smell the roses life would be better and the earth would also. But the earth does go thru changed just like we do. WHAT ARE WE TO DO?

wow… that was interesting!very helpful..im doing a science project. wow.. lol… but i mean like, the galaxies that eat other galaxies, well.. what if someone eats us? i mean like…scary… i dont think people realize that space is like, like a death ticket. dudes. have we ever sent something through the black holes… have you ever wondered if theres another eath, and everyone has a twin, but on that other earth? that would be kick ass! lol…aliens depress me. they make me feel so stupid. cause we know like nothing about them, lol.. but we all no its like so true tho! i mean, we dont even know if theres a species out there that doesnt need lite, water, food ect. we just dont know! im waiting for the day when hubble takes a picture of another planet that looks exactly like planet earth. lol…that would be funny! did you know that like… well this might not be true, but the earths ecuador is bulging out, right? well my teacher says it might be that earth is forming rings! would that just be a ball!

I heard that both the milky way and the andromeda galaxy is larger than previously thought.
In fact, the Milky Way is 500,000 light years larger than andromeda.
Andromda is 5 times larger, while we are 15 times larger.
Andromeda- 1 000 000 lyrs
Milky Way- 1 500 000 lyrs

this helped me a lot because i have a project due and i could find any facts about the milky way
my group totally sucks so i have to do the work all by myself which sucks.
but to make sure this site isn’t a lie i asked my dad who is a police officers to check it out and he said it was fine but i still had to be careful where i get my information from:)!!!

acually we dont know that our galaxy a spiral or what color it is we only know very little. We do not have the technology yet. How can we tell what it looks like from the outside if we have never seen it from the outside its like if u were looking out the window and uve never seen ur house before how will u know what it looks like.

Ok, I know some of these posts are very old, but sheesh! Where do these people come from? For example:

217. al Says:
December 30th, 2008 at 2:29 am

why can’t we even get to the moon; yet be bothered by the fact that an entire galaxy is comming at us and gamma ray bursts are approaching, etc…

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I guess you didn’t notice. We’ve been to the moon several times. I don’t think most rational people are bothered by an entire galaxy “comming” [sic] at us. GRBs however, are a real threat. Not to worry though. Should one actually be headed our way, we’d neither see nor feel it. We’d just die.
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218. Johanna Says:
January 15th, 2009 at 5:38 pm
im waiting for the day when hubble takes a picture of another planet that looks exactly like planet earth. lol…that would be funny!

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Hubble isn’t powerful enough to take a picture of a planet outside our solar system. You’ll be waiting a long time…:)
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did you know that like… well this might not be true, but the earths ecuador is bulging out, right? well my teacher says it might be that earth is forming rings! would that just be a ball!

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Ecuador is bulging out? You do know Ecuador is a country, right? I think you mean the equator. Yes, it bulges, due to the rotation of the Earth. No, it won’t form rings. Just what subject does this teacher teach? Please tell me it isn’t science of any kind…..
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229. Josh Scott Says:
October 5th, 2009 at 1:11 pm

I heard that both the milky way and the andromeda galaxy is larger than previously thought.
In fact, the Milky Way is 500,000 light years larger than andromeda.
Andromda is 5 times larger, while we are 15 times larger.
Andromeda- 1 000 000 lyrs
Milky Way- 1 500 000 lyrs

TAKE THAT ANDROMEDA!!!!!!!!!!!!!!:D

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A little knowledge, especially when it’s wrong, is a dangerous thing….
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231. Anas Says:
December 19th, 2009 at 11:14 am

that was amazing but im not sure if it is all true, you sholud read the english translation of the quran for better help.

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(sigh) I’m not even going to touch that one…..
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p.s. the wolrd isnt going to end in 2012

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Ya think? Even the Mayan calendar doesn’t say it will end in 2012. It says the current Great Cycle, a calendar phase lasting approximately 5125 years, ends then. According to the Mayan traditions, this has happened 5 times in the past, every 5125 years. Since this totals slightly under 26000 years, the Mayans are probably off a bit. Makes me wonder what the Quran says about the Mayans….
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234. aaron dieguez Says:
April 1st, 2010 at 10:08 am

idk if this is correct because i thought that if two galaxys came together they would go supernova and explode not merge together i saw that on the show NOVA

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Galaxies do not go “supernova”. Stars do. Galaxies “collide” for lack of a better word, intermingle, mesh and form a new, larger galaxy. It has happened many times, and if you read the original post, you would have noticed that our galaxy has done this before. Quite possibly many times. I would love to know which episode of NOVA you are referencing.
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Ok, I could have been more, hmmm, kind in my comments. Then again, I could have been a lot more sarcastic Let’s just call it the middle road…

I knew 10 out of 10. The blog helped. Also reading Scientific American and Sky & Telescope for 60 years. Nobody brings in frame-dragging (the Lense-Thirring effect) but I would bet that that provides a better explanation of both the spiral arms and dark matter than the lame conventional (and incomprehensible) explanation. Frame dragging has been proven by Gravity Probe-B and it is really small for the Earth but would be really sizable for supermassive black holes. The idea is that the arms only appear to spiral because the space-time that a galaxy is embedded in is dragged into a rotational motion by the rotating central massive black hole. But then, what do I know? Anyone care to kick this around?

I have the same feelings about the calculations of dark matter… if space time is bent by the wieght of a collapesed massive star to create a black hole, what happens to space time when you apply a less focused but much more massive force to it ? However this would most likely not be enough to elminate 90% of the missing mass but it seems it could influence the calculation…… what are you thoughts ?

Hmm, oops thats what I get get for writing a comment in the middle of the night here…. Well now that I mumbled something I should clarify…. combined wieght of the the entire galaxy as being a less focused but much more massive force than for example a black hole…. sorry about that.

It seems(have no idea for certian) dark matter can be measured/observed (a guess) by observing the gravitational lensing effect. Another observble way is when two galaxy have collided and during a certian phase the dark matter seems to be the mass the galaxys are rotating around (i am sure there are many better technical words for this 😛 and much better explanations and other ways of deducings its presence ) I would suggest that it may not be the super massive black hole in the center of the galaxy but rather the combined effect of the entire galaxy on space time and during this interaction of galaxies combining, space time may be influenced enough that it may be observed. As to speculation into as the precise mechnism….there isnt enough space in the comments here… just a conceptual thought… anway I suppose i didnt make myself that much clearer 😛 was just kicking the idea around as you asked 😀 But maybe some smart astronomer could clarify 😀 or we can kick it around some more 😀